1. Technical Field
The present invention relates to beamforming and, more particularly, to performing channel estimation to determine weights of beaminforming signals.
2. Description of the Related Art
Wireless local area networks (WLANs) have become an important last-mile technology for providing interne access within homes and enterprises. In such indoor deployments, the wireless channel suffers from significant multipath scattering and fading that degrades performance. One way to cope with such multipath effects is through reactive media access control (MAC) layer mechanisms, such as retransmissions or rate adaptation. Recent developments in smart antenna technologies provide a proactive means to alleviate these multipath impairments. Such proactive smart antenna technologies vary with respect to the sophistication of signal processing at the transmitter (Tx) and receiver (Rx). Examples these technologies include Multiple Input Multiple Output (MIMO), Directional antennas (Dir) and beamforming (Bf).
MIMO is a popular strategy that uses multiple antenna elements at the Tx and Rx and leverages the rich scattering nature of the environment to increase the link capacity. In open-loop MIMO schemes, the receiver performs channel estimation to determine the multipath channel, which then uses this information to appropriately process the received signal. In closed-loop MIMO schemes, the estimated channel is known to the Tx as well. Such knowledge of the channel permits the Tx to control the instantaneous amplitude and phase of the signals transmitted on the different antenna elements to exploit the multipath channel.
In directional antennas approaches, used popularly for outdoor scenarios, the transmission pattern is preset to point a main lobe (providing high gain) in the direction of the Rx. Because the transmit pattern is oblivious to channel state information, Dir is affected by multipath propagation and has been shown to be less effective in handling multipath in indoor environments.
In contrast, Beamforming (Bf) is a closed-loop technique that uses the channel information to modify the transmitted signal such that the signals received through the multiple paths are reinforced at the Rx, thereby improving the link Signal to Noise Ratio (SNR). Unlike other MIMO techniques, beamforming does not require multiple elements at the clients and its benefits can be attained even with omni-directional clients.